Originally Posted by Olivier
Depending where you live, it's pretty common not to mention any partners you already have when you are dating someone for the first time. Only once the relationship becomes established you start setting the terms. In regards to sex: Practice safe sex.
I agree that it depends on where you are and the particular people involved what all needs to be disclosed at the early stages of getting know/dating someone, but there is no such thing as safe sex - there's safeR sex. Herpes and HPV can both be spread even with consistent and correct condom usage. Risks of exposure are obviously higher the more partners one has. It would really upset me if someone I was sleeping DIDN'T tell me they were having sex with other people if they were - condoms or not. Whether or not to be exclusive or who/how how many people you're dating is an entirely different conversation than sexual practices. To me, anyway. Granted, it is each individual's responsibility to maintain their own health, but I personally make sure to ask about the basics and volunteer the information about myself before becoming sexual with a new person.
As for the OP - I think the first step is just talking to her. You don't have to offer up any details about your personal life to start getting to know her. I'm a fan of the friends first approach to dating since it seems to create less pressure to "form a relationship" or anything like that. Just see if you two can hang out sometime when she's not working to see if you're still interested. If she seems interested too, THEN you can figure out what you think is best for the situation. Casually dating for a while until you're sure it could lead to something or telling her everything and letting her decide if she wants to pursue you. Taking the time just to hang out with this other woman would also give your gf time to adjust to you spending time with another person to see how she will handle possible future issues.